Interlaced Filtration Barrier

ABSTRACT

A filtration barrier comprises at least one barrier layer which includes polymeric nanofibers interlaced with microfibers, and at least one substrate layer which includes polymeric microfibers. The filtration barrier can be made by electrospinning process.

FIELD OF INVENTION

This invention relates to a polymer-based filtration barrier and amethod of fabricating the same. In particular, the filtration barrierhas a barrier layer and a substrate layer.

BACKGROUND OF INVENTION

A desirable filtration barrier should possess high filtration efficiencyand low pressure drop. Some filtration barriers are made of meltblownfibers having diameters ranging from a few microns to a few tens ofmicrons. Those barriers cannot effectively filter out most micron-sizedparticles and nanoparticles while maintaining a low pressure drop.

SUMMARY OF INVENTION

In the light of the foregoing background, it is an objective of thepresent invention to provide an alternate filtration barrier.

Accordingly, the present invention, in one aspect, provides a filtrationbarrier which includes at least one barrier layer and at least onesubstrate layer. The barrier layer includes a plurality of polymer-basednanofibers interlaced with a plurality of polymer-based microfibers. Thesubstrate layer includes polymer-based microfibers. The barrier layer isattached onto the substrate layer. The nanofibers and/or microfibers ofthe barrier layer bear electrostatic charges.

In an exemplary embodiment of the present invention, the diameter of thepolymer-based nanofibers of the barrier layer ranges from 10 nanometersto 1000 nanometers; the diameter of the polymer-based microfibers of thebarrier layer ranges from 1 micron to 10 microns.

In another exemplary embodiment of the present invention, the filtrationbarrier further includes at least one supporting layer attached to thebarrier layer, and the supporting layer includes polymer-basedmicrofibers.

In another aspect, the invention provides a filtration barrier whichincludes a nanofiber layer and a biocide layer. The nanofiber layerincludes a plurality of polymer-based nanofibers. The biocide layerincludes a plurality of nanofibers made of polymers having reactivegroup and biocides which bind to the reactive group by crosslinker. Thenanofiber layer is attached onto the biocide layer. The amount ofbiocide accounts for 0.5-2 weight percent in the biocide layer, and therange for the porosity of the biocide layer is 90-98%.

In a further aspect, the invention provides a method of fabricating afiltration barrier which includes the step of interlacing nanofiberswith microfibers to form a barrier layer having a pore size of 100-10000nm such that said filtration barrier can block more than 95% ofparticles of 200-400 nm at an air flow of less than 85 L/min, whilemaintaining a pressure drop of less than 35 mm H₂O. Particles of lessthan 200 nm can be blocked by Brownian diffusion while particles of morethan 400 nm can be blocked by impaction, interception, electrostaticattraction and sieving.

In one embodiment, the above method further comprises a step of varyingthe conductivity, viscosity or surface tension of the polymer solution.

In a further embodiment, the above varying step further comprises thestep of adding conductivity-enhancing additives.

In an exemplary embodiment of the present invention, the method furtherincludes a step of attaching the interlaced barrier layer onto asubstrate layer, and the substrate layer includes polymer-basedmicrofibers.

In another exemplary embodiment of the present invention, the nanofibersand microfibers are made from a polymer solution and obtained by varyingthe conductivity, viscosity or surface tension of the polymer solution.

In a further aspect, the invention provides a method of fabricating afiltration barrier, which includes step (1) forming a barrier layercomprising nanofibers interlaced with microfibers by varying at leastone dimension of a thread produced from a polymer solution; and (2)attaching the barrier layer onto a substrate layer to form thefiltration barrier. The thread is produced by electrospinning with thepolymer solution; and the substrate layer comprises a plurality ofmicrofibers.

In a further aspect, the invention provides a method of fabricating afiltration barrier comprising a nanofiber layer and a biocide layer. Thenanofiber layer includes a plurality of non-polar polymeric nanofibersbearing positive electrostatic charges. The biocide layer includes aplurality of nanofibers made of a polymer with a hydroxyl group that cancovalently bind to a biocide with an amino group by a crosslinker.

The filtration barrier of the present invention provides a number ofadvantages, for example, the barrier layer includes an interlacedstructure of nanofibers and microfibers in which additional support tothe filtration barrier can be provided by the interlace withoutcompromising the filtration efficiency of the filtration barrier. Theinventors of the present invention appreciate the fact that althoughfiltration efficiency generally increases with increasing nanofibersurface area, and that the surface area increases with decreasingnanofiber diameter, the nanofiber diameter cannot be reduced extensivelywithout limit for a given material. If one needs to further increase thefiltration efficiency of the barrier comprising nanofibers at a minimumpossible diameter, one has to increase the thickness of the barrier,leading to an increase in pressure drop of the barrier. Therefore, thecurrent invention is the solution to develop a filtration barriercapable of increasing filtration efficiency without causing significantincrease in pressure drop.

The current inventors recognize that nanofibers fabricated via positivevoltage and negative voltage electrospinning bear positive and negativeelectrostatic charges respectively and these charges can be retained forrelatively long times. The current inventors also recognize that thesecharges come from two sources: residual surface charges and excesstrapped charges (ETCs) in nanofibers. Unlike surface charges, ETCs donot dissipate easily as ETCs can accumulate at domain boundaries of apolymer. For semicrystalline polymers, charges can be trapped atcrystalline/amorphous interface. Electrospun nanofibers made ofnon-polar polymers generally retain charges for a longer period of timewhen compared with those made of polar polymers. As most bacteria andviruses are negatively charged, the present solution to this problem isthe formation of nanofibrous barriers bearing positive charges. Thosecharge-bearing barriers can enhance filtration of bacteria and virusesby electrostatic attraction without increasing pressure drop.

On the other hand, electrospun nanofibers can be functionalized withbiocides capable of killing bacteria on contact while most commerciallyavailable filtration barriers merely serve as sieves to filter outairborne contaminants. An ideal barrier should exhibit bactericidalfunction in addition to physical blockage of airborne contaminants inorder to minimize cross contamination. The inventors recognize thatalthough some barriers can exhibit bactericidal property, those barriersoften require leaching of biocides (e.g. silver nanoparticles) away fromthe barriers in order to kill bacteria. The present solution to developa nanofibrous barrier capable of killing bacteria on contact withoutleaching biocides away from the barrier.

Besides, the filtration barrier has a higher filtration efficiency withlower pressure drop as illustrated in the results discussed below.Further, the barrier layer can be made by one set of solution, of whichthe conductivity, viscosity, and surface tension are unstable, so thatthe method of fabricating the barrier layer can be easily scaled up forlarge-scale production. Last but not least, the barrier layer, whenattached to the substrate layer, can maintain its mechanical integrity.Yet, the filtration barrier comprising the substrate layer and thebarrier layer is flexible such that roll-to-roll processing is allowedwithout affecting the mechanical integrity of the barrier layer.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 illustrates a filtration barrier including a barrier layer and asubstrate layer.

FIG. 2 illustrates the SEM image of CA/PEO nanofibers interlaced withmicrofibers (×5000 magnification) according to one embodiment of thepresent invention.

FIG. 3 illustrates a filtration barrier including a barrier layer, asubstrate layer and a supporting layer according to the same embodimentof the present invention.

FIG. 4 illustrates a filtration barrier including five barrier layersand five substrate layers and one supporting layer according to oneembodiment of the present invention.

FIG. 5 illustrates the result of a study on filtration efficiency andpressure drop of the filtration barriers with different numbers ofbarrier layers and substrate layers. The barrier layer includes CA/PEOnanofibers interlaced with microfibers without electrostatic charges.

FIG. 6 illustrates the result of a study on filtration efficiency andpressure drop of different numbers of CA/PEO nanofiber layers.

FIG. 7 illustrates the result of a study on filtration efficiency andpressure drop of the filtration barriers with different numbers ofbarrier layers and substrate layers. The barrier layer includes CA/PEOnanofibers interlaced with microfibers with electrostatic charges.

FIG. 8 illustrates the result of a study on surface potential of onebarrier layer including CA/PEO nanofibers interlaced with microfibersover a period from the end of electrospinning to the start ofmeasurement.

FIG. 9 illustrates the result of a study on surface potential of onebarrier layer including CA/PEO nanofibers over a period from the end ofelectrospinning to the start of measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein and in the claims, “comprising” means including thefollowing elements but not excluding others.

Example 1 Filtration Barrier Including Nanofibers Interlaced withMicrofibers 1.1 Structure of the Filtration Barrier

The first aspect of this invention, as shown in FIG. 1, relates to afiltration barrier 20 including a barrier layer 22 and a substrate layer24. The barrier layer 22 includes an interlaced structure ofpolymer-based nanofibers and polymer-based microfibers in whichnanofibers and microfibers bear electrostatic charges, whereas thesubstrate layer 24 is made of polymer-based microfibers. The barrierlayer 22 is attached onto the substrate layer 24 via mechanicalinterlocking and electrostatic attraction.

In one embodiment, the perimeter of the barrier layer 22 can also beattached onto the perimeter of the substrate layer 24 via ultrasonicwelding.

In one embodiment, the thickness of the barrier layer 22 ranges from 5to 100 microns whereas the thickness of the substrate layer 24 rangesfrom 90 to 200 microns.

FIG. 2 shows the morphology of the interlaced structure of the barrierlayer 22 under Scanning Electron Microscope (×5000 magnification). Inone embodiment, the diameters of the nanofibers range from 20 to 500nanometers while the diameters of the microfibers range from 1 to 3microns.

In another embodiment as shown in FIG. 3, the filtration barrier 30further includes a supporting layer 32 attached to the barrier layer 22.The supporting layer 32 is made of polymer-based microfibers. In oneembodiment, the supporting layer can be made of polyester, nylon,polyethylene, polyurethane, cellulose, polybutylene terephthalate,polycarbonate, polymethylpentene or polystyrene. In another embodiment,the diameter of the supporting layer ranges from 1 micron to 10 mircons.

In a specific embodiment, the barrier layer 22 of the filtration barrier30 as shown in FIG. 3 is made of a combination of CA and PEO at a weightof 0.8 gsm in which both the nanofibers and microfibers are produced byelectrospinning. Electrospinning, a technology capable of formingnanofibers/microfibers, is a promising approach to generate airfiltration barriers having high filtration efficiency at low pressuredrop. The substrate layer 24 is a nonwoven fabric made of 20 gsmpolypropylene (PP) with antistatic treatment, in which the microfibersare produced by spunbonding or meltblowing. Lastly, the supporting layer32 is a nonwoven fabric made of 50 gsm PP, in which the microfibers arealso produced by spunbonding or meltblowing. Nonwoven mesh of nanofibersis a desirable material for filtration because of its high specificsurface area. This property can facilitate trapping of tiny particlesvia various mechanisms such as sieving, interception and Browniandiffusion.

In one embodiment, the substrate layer is a nonwoven fabric made of10-100 gsm polypropylene, and the supporting layer is a nonwoven fabricmade of 40-120 gsm polypropylene.

In yet another embodiment, a filtration barrier can include multiplebarrier layers and substrate layers. For instance, filtration barrier 40as shown in FIG. 4 includes five barrier layers 22, five substratelayers 24 and one supporting layer 32, in which each of the five barrierlayers 22 is attached onto one substrate layer 24, and one of thebarrier layers 42 is sandwiched between a substrate layer 44 and thesupporting layer 32.

1.2 Synthesis of the Filtration Barrier

15 wt % of cellulose acetate (CA) and 0.1 wt % of polyethylene oxide(PEO) were dissolved in dimethylformamide (DMF), and 0.3 wt % ofbenzyltriethylammonium chloride (BTEAC) was added to obtain a polymersolution.

The polymer solution was loaded into an electrospinning system in whichelectrospinning of the polymer solution was performed under thefollowing conditions to form the barrier layer 22: coating time of 2 hr(with basis weight of 0.8 gsm), voltage at 25 kV, working distance of 15cm, flow rate at 0.5 ml/h, needle ID at 0.8 mm, 23° C., and relativehumidity of 60%. Drum collector was used for the electrospinningprocess.

In one embodiment, the working range of applied voltage is 10-50 kV; theworking range of the distance is 10-30 cm; the working range of thesolution flow rate is 0.05-5 ml/h; and the working range of the relativehumidity is 30-80%, and the temperature is 20-30° C.

In one embodiment, the barrier layer formed has a pore size of 200 to5000 nm such that more than 95% of particles of 10 to 800 nm cannot passthrough the barrier while maintaining a pressure drop of 31 mm H2O whenthe air flow is 3 L/min.

The size or diameter of the CA/PEO fibers discharged from theelectrospinner can be varied by adjusting the properties of the polymersolution, for example conductivity, viscosity and surface tension,during electrospinning. For instance, the size or diameter of thedischarged fiber will be smaller if the conductivity of the polymersolution is increased, and the same effect can be observed if theviscosity of the polymer solution is decreased.

In order to make the properties of the polymer solution to beadjustable, a step of adding special conductivity-enhancing additivessuch as organic salts, inorganic salts, hygroscopic species or carbonblack is included. Some examples of the additives are BTEAC, phosphorousacid (H₃PO₄), lithium chloride (LiCl), tetraethylammonium bromide (TEAB)and tetrabutylammonium bromide (TBAB). For example, to unstablize theconductivity of the polymer solution, 0.3 wt % of BTEAC is added to thepolymer solution, such that the conductivity of the polymer solution isfluctuated within a range of 30-32 μS/cm.

The barrier layer produced by electrospinning is directly deposited ontoa substrate layer made of polymer-based microfibers and the barrierlayer is attached onto the substrate layer via mechanical interlockingand electrostatic attraction to form the filtration barrier. In oneembodiment, a supporting layer made of polymer-based microfibers isattached to the barrier layer via ultrasonic welding to provide extrastrength and support to the filtration barrier.

1.3 Study on the Filtration Efficiency and Pressure Drop of theFiltration Barrier

The filtration efficiency and pressure drop on different numbers and/ordifferent types of barrier layers synthesized in the aforementionedmethod were evaluated by passing particles with size ranging from 10-800nm generated from an aerosol generator through the filtration barrier.An aerosol monitor was used at the other end of the filtration barrierto monitor the particles that can pass through.

1.3.1 Barrier Layer Having the Interlaced Structure of Nanofibers andMicrofibers with No Electrostatic Charge

A shown in FIG. 5, the filtration efficiency and pressure drop of thedifferent numbers of the barrier layers of the present inventionsynthesized by the aforementioned method, were compared. The barrierlayer has nanofibers interlaced with microfibers at a weight of 0.8 gsm.The nanofibers interlaced with microfibers have no electrostatic charge.It can be seen that with the increase in the number of barrier layers,the filtration efficiency barrier layers both increase. N95, a verycommonly used respirator which is able to filter 95% contaminants, isused as a control. As further observed, the sample with six barrierlayers has a higher filtration efficiency, but yet a lower pressure dropthan N95.

From the result, it can be seen that an enhanced filtration effect canbe achieved using the barrier layer of the filtration barrier of thepresent invention. In particular, the filtration performance was betterthan that of N95 filter when six barrier layers were used.

1.3.2 Barrier Layer Having Nanofibers Only with No Electrostatic Charge

FIG. 6 shows the filtration efficiency and pressure drop of a barrierlayer having only the CA/PEO nanofibers (i.e. without interlace ofCA/PEO nanofibers and CA/PEO microfibers) at a weight of 0.8 gsm with noelectrostatic charge. On comparing the results shown in FIGS. 5 and 6,it can be seen that the pressure drop is reduced for barrier layerhaving the interlaced structure of the present invention as comparedwith that of the barrier layer without the interlaced structure.

Thus the result shows that barrier layer with the interlaced structureof the present invention could provide a lower pressure drop withoutcompromising the filtration efficiency.

1.3.3 Barrier Layer Having the Interlaced Structure of Nanofibers andMicrofibers with Electrostatic Charge

In this study, the filtration efficiency and pressure drop of thedifferent numbers of the barrier layers of the present invention (i.e.including interlace of nanofibers and microfibers) having electrostaticcharge, synthesized by the aforementioned method, were evaluated and theresult was shown in FIG. 7.

As shown in FIG. 7, the CA/PEO nanofibers interlaced with microfibershave electrostatic charges at a range of 28-32V which were produced byelectrospinning process. On comparing the results shown in FIGS. 5 and7, it can be seen that the barrier layer having electrostatic chargehave a higher filtration efficiency than that of the barrier layerwithout electrostatic charges. Further, sample with four barrier layerswith an electrostatic charge of 32V possessed a higher filtrationefficiency and a lower pressure drop than N95.

Thus the result shows that electrostatic charges on the interlacedstructure can increase the filtration efficiency without compromisingthe pressure drop.

Besides, the surface potential of one barrier layer of the presentinvention (i.e. having interlace of nanofibers and microfibers) at aweight of 0.8 gsm was measured by electrostatic voltmeter (TREK 706B).The surface potential was measured from the end of the electrospinningto 60 days afterwards and shown in FIG. 8. The surface potential wasreduced to zero approximately 20 days after the end of theelectrospinning, while as shown in FIG. 9 for barrier layer havingCA/PEO nanofibers only, the surface potential was reduced to zeroapproximately 10 days after the end of electrospinning. Therefore, theresult shows that the interlaced structure can maintain electrostaticcharge for a longer time than the barrier layer having nanofibers only.

1.4 Electrospinning of Biocide-Loaded Nanofibers that are Interlacedwith Microfibers

10 wt % of CA and 0.1 wt % of PEO were dissolved in DMF; 1 wt % ofchlorhexidine (CHX) as biocide and 1 wt % of organic titanate (TTE) ascross-linker were added into the solution. The biocide-loaded solutionwas prepared into an interlaced structure of nanofibers and microfibersby electrospinning process as mentioned above. In one embodiment, theworking range of the applied voltage is 10-50 kV; the working range ofthe distance is 10-30 cm and the working range of the solution flow rateis 0.1-3 ml/h.

The above example is given by way of illustration of the presentinvention but should not be considered to limit the scope of theinvention. For example, the percent of CA and PEO and other componentsby weight in the solution is not limited to that in the above example.The polymer solution can include CA in the range of 10-20 by weightpercent; PEO in the range of 0.05-0.2 by weight percent;benzyltriethylammonium chloride (BTEAC) in the range of 0.1-0.3 byweight percent, and DMF in the range of 80-90 by weight percent.Further, the polymer used to produce the interlaced structure ofnanofibers and microfibers of the barrier layer is not limited tocombination of CA and PEO. The polymer used can be collagen, elastin,gelatin, fibrinogen, fibrin, alginate, cellulose, silk fibroin, chitosanand chitin, hyaluronic acid, dextran, wheat gluten,polyhydroxyalkanoates, laminin, nylon, polyacrylic acid (PA),polycarbonate (PC), polybutylene terephthalate (PBT), polyurethane (PU),poly(ethylene vinyl acetate) (PEVA), polycaprolactone (PCL),polyglycolic acid (PGA), poly(lactic acid) (PLA),poly(lactic-co-glycolic acid) (PLGA), polyacrylonitrile (PAN),polystyrene (PS), polyvinyl alcohol (PVA), cellulose acetate (CA),polyethylene oxide (PEO) or any combination thereof. Also, thenanofibers and microfibers of the barrier layer can be made of non-polarpolymers.

Although polypropylene (PP) is disclosed in the above example asingredient for making the substrate layer and the supporting layer, itis clear to one skilled in the art that other polymers such aspolyester, nylon, polyethylene, polyurethane, cellulose, polybutyleneterephthalate, polycarbonate, polymethylpentene and polystyrene can alsobe used.

The filtration barrier may contain multiple barrier layers, substratelayers and supporting layers, and optionally biocide layer(s).

The percentages of nanofibers and microfibers by weight in theinterlaced structure of nanofibers and microfibers account for 60%-70%,and 30%-40%, respectively. The range of porosity of the interlacedstructure is 80-98%.

In a preferred embodiment, the nanofibers account for 65% by weight inthe interlaced structure, and the microfibers account for 35% by weightin the interlaced structure.

The diameter of microfibers of the substrate layer and the supportinglayer ranges from 0.002-0.02 mm and 0.005-0.05 mm respectively. Thethickness of a single barrier layer, a single substrate layer and asingle supporting layer ranges from 5-100 microns, 90-200 microns, and150-400 microns, respectively.

The biocide used in the filtration barrier can be chlorhexidine (CHX),copper oxide, silver nanoparticles, calcium peroxide, N-halamines, andantibiotics. The cross-linker can be titanium triethanolamine, organictitanate, glutaraldehyde or genipin.

For the electrospinning process described above, the working conditionsthereof include: applied voltage of 15-30 kV, working distance of 10-30cm, solution flow rate at 0.1-5 ml/h, coating time of 0.5-5 hr. Inaddition, drum collector or plate collector can be used for theelectrospinning process.

Example 2 A Filtration Barrier Including a Nanofiber Layer and a BiocideLayer

In another aspect of this invention, a filtration barrier is describedwhich includes two layers, nanofiber layer and biocide layer, attachedto each other. The nanofiber layer is positioned distal to the air-flowdirection and includes polymer-based nanofibers bearing positiveelectrostatic charge. The biocide layer is positioned proximal to theair-flow direction and includes polymer-based nanofibers crosslinkedwith biocides, in which a first reactive group in the polymer cancovalently bind to a second reactive group of the biocide via acrosslinker.

In one embodiment, the nanofiber layer and biocide layer may further beattached to several fibrous layers.

On the biocide layer, trap particles can be trapped and kill bacteriacan be killed on contact while with the presence of positive charges onthe barrier layer, filtration of negatively charged particles such asmost bacteria and viruses can be further facilitated. Positive voltageelectrospinning is employed to make the nanofibers bearing positiveelectrostatic charges. The filtration barrier can be used for waterfiltration, provided that the polymer used does not disintegrate inwater.

A method of fabricating a filtration barrier includes coating nanofiberlayer onto biocide layer. Alternatively, the filtration barrier can befabricated by first forming the barrier layer comprising positivelycharged nanofibers and the layer comprising biocide-crosslinkednanofibers separately, followed by assembling them together.

In one embodiment, the nanofiber layer was attached onto the biocidelayer, which was made by electrospinning system. A CA/PEO solution wasblended with CHX and TTE so that biocide-crosslinked nanofibers can bemade via a one-step process. The biocide layer was used as a substrateto collect nanofibers bearing positive electrostatic charges which weremade by electrospinning process.

In one embodiment, the percentage of CA by weight in the CA/PEO solutioncan be in the range of 10-20 by weight percent, and that of PEO can be0.05-0.2 by weight percent.

In another embodiment, the nanofiber layer and the biocide layer wereseparately made by electrospinning. Both layers were assembled togetherto form a filtration barrier. Multiple nanofiber layers and biocidelayers can be assembled together to form a filtration barrier.

The nanofiber layer and the biocide layer can be assembled with othernonwoven layers including polypropylene meltblown microfibers.

The filtration barrier can filter out at least 95% of sodium chlorideaerosol having the most penetrating particle size at an airflow rate of85 L/min while the pressure drop does not exceed 35 mm water.

The exemplary embodiments of the present invention are thus fullydescribed. Although the description referred to particular embodiments,it will be clear to one skilled in the art that the present inventionmay be practiced with variation of these specific details. Hence thisinvention should not be construed as limited to the embodiments setforth herein.

For example, the polymer used to make the nanofiber layer or the biocidelayer has the ability to retain residual charges for relatively longtimes. The polymer used in the nanofiber layer or in the biocide layercan be made of non-polar polymer; in another embodiment, the polymer canbe collagen, elastin, gelatin, fibrinogen, fibrin, alginate, cellulose,silk fibroin, chitosan and chitin, hyaluronic acid, dextran, wheatgluten, polyhydroxyalkanoates, laminin, nylon, polyacrylic acid (PA),polycarbonate (PC), polybutylene terephthalate (PBT), polyurethane (PU),poly(ethylene vinyl acetate) (PEVA), polycaprolactone (PCL),polyglycolic acid (PGA), poly(lactic acid) (PLA),poly(lactic-co-glycolic acid) (PLGA), polyacrylonitrile (PAN),polystyrene (PS), polyvinyl alcohol (PVA), cellulose acetate (CA),polyethylene oxide (PEO) and combination thereof.

The filtration barrier may contain multiple nanofiber layers and biocidelayers. The diameters of the nanofibers of the nanofiber layer rangefrom 50-700 nm, and the diameters of the nanofibers of the biocide layerrange from 100-900 nm. The thickness of the nanofiber layer or thebiocide layer is in the range of 5-100 microns.

In one embodiment, the nanofibers of the biocide layer exhibit anaverage diameter ranging of 50-900 nm. The thickness of the biocidelayer and the nanofiber layer range of 100-500 mm. The nanofiber layerretains residual positive charges for a period of time ranging from 90days to 1 year.

In the biocide layer, the first reactive group of the polymer can behydroxyl group or amino group, whereas the second reactive group of thebiocide can be an amino group or hydroxyl group.

Further, the biocide can be chlorhexidine (CHX), copper oxide, silvernanoparticles, calcium peroxide, N-halamines, or antibiotics, etc. Thecross-linker can be used to bind the reactive groups of the biocide tothe reactive groups of the polymer such that the biocide-crosslinkednanofibrous layer can kill bacteria on contact without leaching thebiocide away from the polymer. The crosslinker can be titaniumtriethanolamine or organic titanate, etc.

For small-scale production of the nanofibers of both layers, aneedle-electrospinning system can be used. For large-scale production ofnanofibers, a needleless-electrospinning system can be used.

Further, for the fabrication of the biocide layer, no matter whichelectrospinning technique is employed, the biocide-crosslinkednanofibers can be made via a one-step process or a post-electrospinningtreatment. For the one-step process, a polymer solution is blended witha biocide and a crosslinker so that biocide-crosslinked nanofibers canbe electrospun in a single-step process. For the post-electrospinningtreating, a polymer solution is electrospun into nanofibers first,followed by binding a crosslinker to the polymer and then binding abiocide to the crosslinker.

For the electrospinning process, the working conditions thereof include:applied voltage of 10-50 kV, working distance of 10-30 cm, solution flowrate at 0.1-5 ml/h.

1.-27. (canceled)
 28. A method of fabricating a filtration barrier, comprising: obtaining a polymer solution that includes cellulose acetate (CA) and polyethylene oxide (PEO); electrospinning the polymer solution onto a substrate layer to form an interlaced structure of nanofibers and microfibers so that the nanofibers and the microfibers form a barrier layer; and attaching the barrier layer and the substrate layer to obtain the filtration barrier, wherein the nanofibers account for 60%-70% by weight in the barrier layer and the microfibers of the interlaced structure account for 30%-40% by weight in the barrier layer.
 29. The method of claim 28, wherein the CA accounts for 10%-20% by weight in the polymer solution.
 30. The method of claim 28, wherein the PEO accounts for 0.05%-0.2% by weight in the polymer solution.
 31. The method of claim 28, wherein the polymer solution includes benzyltriethylammonium chloride that accounts for 0.1%-0.3% by weight in the polymer solution.
 32. The method of claim 28, wherein the polymer solution includes dimethylformamide (DMF) that accounts for 80%-90% by weight in the polymer solution.
 33. The method of claim 28 further comprising: varying a conductivity, viscosity or surface tension of the polymer solution during electrospinning the polymer solution onto the substrate layer so that the interlaced structure of nanofibers and microfibers is formed.
 34. The method of claim 28 further comprising: adding a conductivity-enhancing additive into the polymer solution to vary a conductivity of the polymer solution.
 35. The method of claim 28 further comprising: adding a conductivity-enhancing additive into the polymer solution to vary a conductivity of the polymer solution, wherein the conductivity-enhancing additive is benzyltriethylammonium chloride.
 36. The method of claim 28, wherein the substrate layer is a nonwoven fabric made of 10-100 gsm polypropylene.
 37. The method of claim 28, wherein a diameter of the nanofibers is 10 nanometers to 1000 nanometers, and a diameter of the microfibers in the barrier layer is 1 micron to 10 microns.
 38. The method of claim 28 further comprising: attaching the barrier layer to a supporting layer, wherein the barrier layer is sandwiched between the supporting layer and the substrate layer.
 39. The method of claim 28 further comprising: attaching the barrier layer to a supporting layer, wherein the barrier layer is sandwiched between the supporting layer and the substrate layer, the supporting layer is a nonwoven fabric made of 40-120 gsm polypropylene.
 40. The method of claim 28 further comprising: adding a biocide and a cross-linker to the polymer solution, wherein the biocide is cross-linked to the barrier layer, the biocide is chlorhexidine (CHX), and the cross-linker is organic titanate (TTE).
 41. A method of fabricating a filtration barrier, comprising: obtaining a polymer solution that includes a polymer; electrospinning the polymer solution to form a barrier layer, wherein the barrier layer includes an interlaced structure of nanofibers and microfibers; and attaching the barrier layer to a substrate layer to obtain the filtration barrier, wherein the nanofibers and the microfibers of the barrier layer bear electrostatic charges, the nanofibers account for 60%-70% by weight in the barrier layer and the microfibers of the interlaced structure account for 30%-40% by weight in the barrier layer, and the polymer is selected from a group consisting of collagen, elastin, gelatin, fibrinogen, fibrin, alginate, cellulose, silk fibroin, chitosan and chitin, hyaluronic acid, dextran, wheat gluten, polyhydroxyalkanoates, laminin, nylon, polyacrylic acid, polycarbonate, polybutylene terephthalate, polyurethane, poly(ethylene vinyl acetate), polycaprolactone, polyglycolic acid, poly(lactic acid), poly(lactic-co-glycolic acid), polyacrylonitrile, polystyrene, polyvinyl alcohol, cellulose acetate, polyethylene oxide and any combination thereof.
 42. The method of claim 41, wherein the polymer of the interlaced structure is a combination of cellulose acetate (CA) and polyethylene oxide (PEO).
 43. The method of claim 41, wherein the barrier layer is attached onto the substrate layer via mechanical interlocking or electrostatic attraction.
 44. The method of claim 41, wherein a perimeter of the barrier layer is attached onto a perimeter of the substrate layer via ultrasonic welding.
 45. The method of claim 41, wherein a conductivity, viscosity or surface tension of the polymer solution is varied during electrospinning the polymer solution onto the substrate layer so that the interlaced structure of nanofibers and microfibers is formed.
 46. The method of claim 41 further comprising: adding a conductivity-enhancing additive into the polymer solution to vary a conductivity of the polymer solution, wherein the conductivity-enhancing additive is benzyltriethylammonium chloride.
 47. The method of claim 41 further comprising: attaching a supporting layer to the barrier layer, wherein the barrier layer is sandwiched between the supporting layer and the substrate layer. 